Typically, a digital imager array includes a focal plane array of pixel cells, each one of the cells including a photoconversion device such as, e.g., a photogate, photoconductor, or a photodiode. In a complementary metal oxide semiconductor (CMOS) imager a readout circuit is connected to each pixel cell which typically includes a source follower output transistor. The photoconversion device converts photons to electrons which are typically transferred to a floating diffusion region connected to the gate of the source follower output transistor. A charge transfer device (e.g., transistor) can be included for transferring charge from the photoconversion device to the floating diffusion region. In addition, such imager cells typically have a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. The output of the source follower transistor is a voltage output on a column line when a row select transistor for the row containing the pixel is activated.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are herein incorporated by reference in their entirety.
In a typical digital CMOS imager pixel (FIG. 1), when incident light strikes the surface of a photodiode 49, electron/hole pairs are generated in the p-n junction (between regions 21 and 23) of the photodiode 49. The generated electrons are collected in the n-type region 23 of the photodiode 49. The photo charge moves from the initial charge accumulation region to a charge collection region 16, typically a floating diffusion region, or it may be transferred to the floating diffusion region via a transfer transistor 26. The charge at the floating diffusion region 16 is typically converted to a pixel output voltage by a source follower transistor (not shown).
Band gap refers to the energy levels separating valence bands and conductive bands. Different materials may have indirect or direct band gap characteristics. For example, silicon has indirect band gap characteristics. Due to the presence of the indirect band gap in silicon, photons have long absorption lengths in silicon compared to direct band gap materials like GaAs or InP. At infrared wavelengths (700 nm), silicon has an absorption coefficient of 3×103 cm−1, which corresponds to an absorption length of slightly more than 3.0 μm. This necessitates a very large photodiode thickness in order to obtain a reasonable response. A large photodiode, however, results in poor bandwidth due to large transit times needed for carrier collection.
In the conventional pixel of FIG. 1, a large amount of incident light of longer wavelengths are not absorbed by the photodiode 49, leading to decreased quantum efficiency of the pixel. Infrared light, for example, has a long wavelength and penetrates deep into a pixel cell. Infrared light sensors typically have a high aspect ratio (depth relative to width) and increased cross talk (i.e., where charge carriers from one pixel travel to adjacent pixels). In addition, photons may be lost due to recombination in the substrate and/or photons may collect in the contact to the substrate. If the photons travel under the STI region 55 (FIG. 1), they may affect adjacent pixels, also creating cross talk.
Fabry-Perot resonant cavities have been used in other systems, such as lasers, to build up large field intensities at specified resonant frequencies and to act as spatial and frequency filters. In a resonant cavity, a pair of parallel polished planes act like mirrors to create resonance. What is needed, is an imager that can capture longer wavelengths of light (e.g., 650-750 nm or longer) with improved quantum efficiency and without increased cross talk, using a resonant cavity.